Does inactivity cause overweight, or is it the other way around?

November 1, 2011

A study in more than 200 boys and girls undermines the common belief that a lack of physical activity leads to childhood obesity.

A longitudinal study in more than 200 boys and girls undermines the common belief that a lack of physical activity leads to childhood obesity. Rather, the investigation showed the reverse: Fatness leads to physical inactivity.

For 4 consecutive years, beginning when study participants were aged 7 through 10 years, investigators in the United Kingdom made measurements of the children's level of physical activity and percentage of body fat. Physical activity was measured using an accelerometer that the children wore for 7 consecutive days (5 school days and 2 weekend days) at each annual time point. Whole body fat percentage was measured by dual energy x-ray absorptiometry. Body mass index (BMI) and waist circumference also were measured.

Although body fat percentages predicted changes in physical activity during the following years, physical activity levels did not predict subsequent changes in body fat percentage over the same follow-up period. For example, a high body fat percentage at age 7 predicted a decrease in physical activity from 7 to 10 years, but a high level of physical activity at 7 years did not predict a decrease in body fat percentage. Similar associations were seen when BMI and waist circumference were used to evaluate fatness, but these associations were weaker than those based on body fat percentage. Although measures of fatness and physical activity differed between the sexes, the strength of association between the variables did not (Metcalf BS, et al. Arch Dis Child. 2011;96[10]:942-947).

The advantage of this study is its longitudinal design compared with the cross-sectional design of most studies showing association between decreased physical activity and obesity. This design allows the investigators to build a stronger case for cause and effect and a strong case for direction of causality. If they are right, and obesity causes decreased physical activity rather than decreased physical activity causing obesity, these findings would have significant implications for management of both individual and community-wide efforts to prevent and treat obesity. Maybe "eat less, do more" needs to be simplified to "eat less." -Michael Burke, MD